The present invention relates generally to the field of fuel burning for power and steam generation and, in particular, to a new and useful bubble cap assembly for supplying a fluidizing medium to a fluidized bed.
Circulating Fluid Bed (CFB) technology has found popularity as an attractive way to burn solid fuels to provide steam and power. This popularity is evident both domestically and abroad. In a combustion setting, CFB technology provides good combustion efficiency with low emissions and provides fuel flexibility because it is well suited for burning a wide range of solid fuels such as coal, waste coal, anthracite, lignite, petroleum coke, and agricultural waste. Consequently, it has emerged as an environmentally acceptable technology for utility and industrial applications.
In a circulating fluidized-bed furnace or boiler, the bed material, which normally comprises crushed fuel, limestone and ash, is suspended in the stream of air at about 60-70% of the total amount of the air needed for stoichiometric combustion. The bottom of the bed is supported by water cooled membrane walls with air nozzles which distribute the air uniformly. The fuel and limestone sorbent (for sulfur capture) are fed into the lower bed. In the presence of fluidizing air, the fuel and limestone quickly and uniformly mix under the turbulent environment and behave like a fluid. Carbon particles in the fuel are exposed to the combustion air. The balance of combustion air is introduced at the top of the lower, dense bed as secondary air. This staged combustion limits the formation of nitrogen oxides (NOx).
The bed fluidizing air velocity is greater than the terminal velocity of most of the particles in the bed and thus the fluidizing air elutriates the particles upwardly through the combustion chamber to U-beam separators at the furnace exit. The solids captured by these U-beams, including any unburned carbon and unutilized calcium oxide (CaO), are returned directly back into the combustion chamber without passing through an external recirculation. This internal solids recirculation provides longer residence time for the fuel and limestone, resulting in good combustion and improved sulfur capture.
A fluidized bed apparatus is taught in U.S. Pat. No. 4,648,969 to Swanson, which provides a bed plate for use in controlling the flow of fluid through a fluidized bed in a fluid chamber. The fluidized bed apparatus comprises a fluid chamber having an inlet adjacent the bottom of the chamber and an outlet adjacent the top of the chamber. A bed of particulate material, namely carbon particles, is fluidized with cyanide solution and exits through the outlet, while gold and silver are absorbed by the carbon bed. The bed plate is also provided with a plurality of spaced apart bubble caps which include a vertical passageway extending through the bed plate and deflecting the fluid to be discharged.
U.S. Pat. No. 5,161,471 to Piekos teaches an apparatus for reburning ash material of a previously burned primary fuel. A combustion vessel is disclosed which has an inlet for receiving ash material from a primary combustion unit such as a boiler or a furnace and an outlet for discharging the products of the combustion from the vessel. Combustion air is introduced into the reburn vessel to provide a source of underfire and overfire source of combustion air for the bubbling bed and the upwardly moving components rising out of the bubbling bed.
U.S. Pat. No. 5,141,047 to Geoffrey teaches a fluidized bed heat exchanger that includes vertically extending spaced apart tubes for containing an internal fluid flowing in a heat transfer relationship with the walls of the tube. A containment housing surrounds the tubes and contains a flow of fluidized solid particulates moving through a heat exchange chamber around the exterior of the tubes. Fluidizing gas is directed into the solid particulates via a gas plenum chamber. A bubble cap is provided around each tube for preventing the solid particulates from passing into the plenum chamber while permitting the injected fluidized gas to flow into the heat exchange chamber.
U.S. Pat. Nos. 5,455,011, 5,543,117, and 5,632,858 to Kitto teach a system in which fuel and air are combusted in an injector and mixed with steam to form a combustion product and steam mixture. The mixture is injected into a material bed. The combustion and mixing is separated from the bed material by being confined within the injector. The injector is a bubble cap having at least one hole or an injector made of a ceramic porous material.
Bubble caps are widely used in fluidized bed technology to supply the fluidizing medium, such as fluidizing air, into a fluidized bed. The medium should be evenly distributed over a specified bed area, while preventing the bed particles from backsifting into the fluidizing medium supply source, such as a windbox of a fluidized bed furnace, at all times including those periods when the fluidizing medium supply is shut down.
FIG. 1A illustrates a known distributor plate construction, generally referred to as 100, for a CFB which employs a plurality of bubble cap assemblies 150. Portions of the CFB furnace or combustor walls are omitted for clarity; however, a rear furnace wall 105 and side furnace wall 110, advantageously made of fluid-cooled tubes, are shown. The distributor plate 100 is also comprised of fluid-cooled tubes 115 which convey a working fluid, typically a water or water-steam mixture, from an inlet header 120 to the furnace walls. The horizontal tubes 125 forming the distributor plate 100 are spaced from one another but interconnected by steel membrane through which a plurality of apertures are provided. Beneath the tubes 125 is a plenum region 130. The bubble cap assemblies 150 are connected to the aforementioned apertures in the distributor plate 100 and deliver a gaseous fluid under pressure provided into the plenum or windbox region 130 into the bed of granular material (not shown) provided onto the upper portion of the distributor plate 130 to fluidize the granular material and create the fluidized bed in the fluidized bed region 140.
A typical bubble cap assembly 150 is illustrated in FIG. 1B. As shown, each of these known bubble cap assemblies 150 is comprised of a bubble cap proper 155, and a supply pipe 160, typically referred to as the stem 160, which fluidically interconnects the windbox region 130 with the fluidized bed region 140. Fluidizing gas is conveyed upwardly along the stem 160 into the bubble cap 155, from which it is distributed to the fluidized bed via plural outlet holes 165. Jets of fluidizing gas exiting from the outlet holes 165 penetrate into the bed providing its fluidization in the area around each the bubble cap 155. The outlet holes 165 are provided so as to direct the exiting jets of fluidizing gas downwardly toward the distributor plate 100. This feature (along with the outlet hole configuration, i.e. a specified minimum length over diameter ratio) assures that no backsifting occurs when the fluidizing gas supply is shut down and the bed of granular material collapses onto the distributor plate 100.
In order to provide even distribution and good mixing of the fluidizing air in the bed, the air jet velocity from the outlet holes 165 may be close to or even exceed 200 ft/sec. Combined with the often erosive nature of the bed particles, this may result in substantial rates of wear of the bubble caps 155. For a CFB furnace, both the bubble caps 155 and the stems 160 are typically made of stainless steel to withstand bed temperatures of about 1600° F. when the fluidizing medium is shut down and the bubble cap assembly 150 temperature approaches that of the bed material. Periodic replacement of worn stainless steel bubble caps may present a substantial maintenance expense for such a CFB unit.
An improved, reduced maintenance bubble cap assembly which would overcome these and other problems would thus be welcomed by the industry.